JOURNAL OF ELECTRONIC MATERIALS
	ABSTRACTS Volume 25, Number 6, June 1996 This Month Featuring: Regular Issue Papers; Letters Section; Editorial Announcement. View June 1996 Table of Contents.

REGULAR ISSUE PAPERS

KEY WORDS Buffer layers, Mg2Si, Si, thermal evaporation

The formation of Mg₂Si(100), a_o = 6.39Å, on Si(100) substrates has been investigated. Mg was first evaporated onto Si(100) surfaces and Mg₂Si (100) films were formed in a subsequent annealing process. The Mg₂Si layers were characterized by x-ray diffraction and transmission electron microscopy analysis. Optical and scanning electron microscopy analysis show the surface morphology to be smooth. The films are stable under thermal cycling and exhibit low resistivity. Epitaxial films of Mg₂Si on Si(100) could be an ideal substrate for mercury cadmium telluride and antimonide based III-V semiconductor for mid-infrared devices because of its close lattice matching (the lattice misfit factor is less than 1.5%).

Magnetic Properties of Nickel Filament Polymer-Matrix Composites
XIAOPING SHUI and D.D.L. CHUNG
Composite Materials Research Laboratory, State University of New York at Buffalo, Buffalo, NY 14260-4400.

KEY WORDS Composite, fibers, filaments, hysteresis energy loss, magnetic, nickel, polymer

The magnetic properties of polyethersulfone-matrix composites with 3-19 vol.% polycrystalline nickel filaments (0.4 µm diam) were investigated. These filaments were found to exhibit hysteresis energy loss 10800 J/m³ of nickel and coercive force 16.9 kA/m, compared to corresponding values of 4930 J/m³ and 4.7 kA/m for 2 um diam polycrystalline nickel fibers, 1020 J/m³ and 0.5 kA/m for 20 µm diam polycrystalline nickel fibers, and 1280 J/m³ and 2.3 kA/m for solid polycrystalline nickel.

High-Performance, Transparent Conducting Oxides Based on Cadmium Stannate
T.J. COUTTS, X. WU, W.P. MULLIGAN, and J.M. WEBB
National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401.

KEY WORDS Cadmium stannate, transparent conducting oxides

We discuss the modeling of thin films of transparent conducting oxides and we compare the predictions with the observed properties of cadmium stannate. Thin films of this material were deposited using radio-frequency magnetron sputtering. The Drude free-carrier model is used to model the optical and electrical properties. The model demonstrates the need for high mobilities. The free-carrier absorbance in the visible spectrum is used as a comparative figure-of-merit for cadmium stannate and tin oxide. This shows that free-carrier absorbance is much less in cadmium stannate than in tin oxide. X-ray diffraction shows that annealed films consist of a single-phase spinel structure. The post-deposition annealing sequence is shown to be crucial to forming a single phase, which is vital for optimal optical and electrical properties. The films are typically high mobility (up to 65 cm² V^-1 s^-1) and have carrier concentrations as high as 10²¹ cm^-3. Resistivities are as low as 1.3 10^-4 cm, the lowest values reported for cadmium stannate. Atomic force microscopy indicates that the root-mean-square surface roughness is approximately +/-15Å. Cadmium stannate etches readily in both hydrofluoric and hydrochloric acid, which is a commanding advantage over tin oxide.

Highly Strained In_0.35Ga_0.65As/GaAs Layers Grown by Molecular Beam Epitaxy for High Hole Mobility Transistors
MAKOTO KUDO, TOMOYOSHI MISHIMA, HIDETOSHI MATSUMOTO, ISAO OHBU, and TAKUMA TANIMOTO
Central Research Laboratory, Hitachi, Ltd., Kokubunji, Tokyo 185, Japan.

KEY WORDS Double side doped structure, HHMT, highly strained InGaAs, MBE, single side doped structure

We have grown highly strained In_0.35Ga_0.65As layers on GaAs substrates by molecular beam epitaxy to improve the performance of high hole mobility transistors (HHMTs). The mobility and sheet hole concentration of double side doped pseudomorphic HHMT structures at room temperature reached 314 cm²/V.s and 1.19 x 10¹² cm^-2, respectively. Photoluminescence measurements at room temperature show good crystalline quality of the In_0.35Ga_0.65As layers. This study suggests that the performance of HHMTs can be improved by using high-quality In_0.35Ga_0.65As layers for the channel of double side doped heterostructures pseudomorphically grown on GaAs substrates.

Molecular Beam Epitaxial Growth of Strained AlGaInAs Multi-Quantum Well Lasers on InP
M.J. MONDRY,¹ E.J. TARSA,² and L.A. COLDREN¹
1--Department of Electrical and Computer Engineering, University of California, Santa Barbara, CA 93106. 2--Materials Department, University of California, Santa Barbara, CA 93106.

KEY WORDS AlGaInAs, interface recombination, molecular beam epitaxy (MBE), multi-quantum well (MQW) laser

State of the art transparency currents as low as 41 A/cm² per well have been achieved in strained AlGaInAs multi-quantum well (MQW) 1.5 µm lasers. Grown by solid source molecular beam epitaxy, broad area lasers with seven quantum wells exhibit threshold current densities of less than 900 A/cm² for a 300 µm device length, comparable to the best results in this material system by any growth technology. The key to this threshold current density reduction is the optimization of the quantum well width. Experimentally, we found that threshold current densities can be reduced by a factor of two by using MQW active regions with wider wells which we attribute to a reduction in the nonradiative recombination and improved electron-hole overlap. High resolution x-ray diffraction, photoluminescence, and broad area lasers were used to characterize the MQW active regions.

In-Situ Spectroscopic Reflectometry for Polycrystalline Silicon Thin Film Etch Rate Determination During Reactive Ion Etching
TYRONE E. BENSON, LEONARD I. KAMLET, PETE KLIMECKY, and FRED L. TERRY, JR.
Department of Electrical Engineering & Computer Science and the Center for Display Technology & Manufacturing, The University of Michigan, Ann Arbor, MI 48109-2122.

KEY WORDS In-situ thin film metrology, polysilicon, reactive ion etching (RIE), reflectometry

Accurate film thickness monitors are important for the development of real-time feedback control of dry etch processes and are very useful for run-to-run process control and process diagnostics. Technologically important films such as polycrystalline Si, which can have process-dependent refractive indices and/or surface roughness, pose significant challenges for low-cost, high-speed film thickness measurement systems. We have used spectroscopic reflectometry (SR) to make accurate in-situ, high-speed film thickness measurements during plasma etching of polycrystalline Si. The SR system determines the film thickness using a least squares regression algorithm that fits the theoretical reflectance to the experimental reflectance vs wavelength data. We have included physically based models for the variation of the polycrystalline Si bulk refractive indices and surface roughness in the fitting procedure. The parameters of the refractive index models are adjusted at the beginning of each run to account for wafer-to-wafer variations without the use of additional ex-situ measurements. We have used ex-situ spectroscopic ellipsometry to validate the models used and to check the accuracy of our SR measurements. Currently, our SR system can acquire data in 40 ms and the software can calculate the polycrystalline Si thickness in less than 55 ms per measurement, so that a new film thickness and etch rate estimate can be obtained in less than 100 ms. The methods used for analysis of polycrystalline Si are also directly useful for improving the accuracy of microscope-based spectral reflection measurement systems commonly used for in-line measurements. Using similar optical modeling concepts, the SR technique can also be used to accurately measure film thicknesses and etch rates of other thin films with process-dependent optical constants, such as deposited dielectrics and compound semiconductors.

InP-Based Multiple Quantum Well Structures Grown with Tertiarybutylarsine (TBA) and Tertiarybutylphosphine (TBP): Effects of Growth Interruptions on Structural and Optical Properties
A.L. HOLMES, JR., M.E. HEIMBUCH, G. FISH, L.A. COLDREN, and S.P. DENBAARS
University of California, Santa Barbara, Electrical Engineering Department, Santa Barbara, CA 93016.

KEY WORDS GaInAsP, metalorganic chemical vapor deposition (MOCVD), multiple quantum well (MQW), photoluminescence (PL), tertiarybutylarsine (TBA), tertiarybutylphosphine (TBP), x-ray diffraction (XRD)

In this paper, we investigate the effect of interfacial layers on GaInAs(P)/GaInAsP and GaInAs/InP multiple quantum well structures with x-ray diffraction and photoluminescence. We observe a decrease in the room temperature and low temperature photoluminescence intensity as the number of periods is increased which we attribute to the interfaces. Furthermore, different growth interruption schemes show that decomposed As species from TBA have an effect on the structural and optical quality of these structures at both the lower and upper interfaces due to As carry-over. The effect of this carry-over is shown in structural measurements and laser diode results.

Interfacial Properties of YBa₂Cu₃0_7-x Thin Films on Al₂0₃ Substrates Prepared by Pulsed Laser Deposition
SANG YEOL LEE¹ and HYUNG-HO PARK²
1--Department of Electrical Engineering, Yonsei University, Sinchondong 134, Seodaemun-ku, Seoul, 120-749, Korea. 2--Department of Ceramic Engineering, Yonsei University, Sinchondong 134, Seodaemun-ku, Seoul, 120-749, Korea.

KEY WORDS Buffer layer, interfacial properties, pulsed laser deposition, superconducting thin film

The interfaces of YBa₂Cu₃0_7-x (YBCO) superconducting thin films grown on () r-plane Al₂0₃ by pulsed laser deposition have been investigated by a transmission electron microscopy and an Auger electron spectroscopy depth profile. We used the PrBa₂Cu₃0_7-x (PBCO) buffer layer to prevent the interdiffusion and compared the interfaces of YBCO/Al₂0₃ and YBCO/PBCO/Al₂0₃. The intermediate layer in the YBCO film deposited on bare sapphire is visible between the film and the substrate but no boundary layer in the film grown on PBCO buffered sapphire was observed directly by the cross-section image of TEM. The thickness of the intermediate layer in the film on bare sapphire is about 30 nm. This result of TEM observation is consistent with that of AES depth profile.

Thermomechanical Deformation of 1 µm Thick Cu-Polyimide Line Arrays Studied by Scanning Probe Microscopy
D.V. ZHMURKIN,¹ T.S. GROSS,¹ L.P. BUCHWALTER,² and F.B. KAUFMAN³
1--Department of Mechanical Engineering, University of New Hampshire, Durham, NH 03824. 2--IBM T.J. Watson Research Center, Yorktown Heights, NY 10598. 3--Cabot Corporation, Microelectronic Materials Division, 500 Commons Drive, Aurora, IL 60134.

KEY WORDS Atomic force microscope, coefficient of thermal expansion, interconnects, polyimide, thermal expansion

Due to the lower dielectric constant than the currently used Si0₂ dielectric, several polyimides are being considered for use in multi-layer high density interconnects. However, the mismatch in out-of-plane coefficient of thermal expansion (CTE) between the Cu conductor and most spin-coated polyimides may cause interfacial failures. A scanning probe microscope was used to estimate the out-of-plane CTE for several 1 µm thick, spin-coated polyimides and to observe thermally induced deformation of Cu-polyimide test structures on Si. The change in relative height of arrays of parallel Cu and polyimide lines of various aspect ratios were imaged in air at room temperature and 97°C. Linear elastic, generalized plane strain finite element models for different out-of-plane CTEs were used to estimate the CTE from the profile changes. It was observed that narrow (¾=1 µm) polyimide lines are more constrained from expansion than wider lines.

Thermodynamic Assessments of the Sn-In and Sn-Bi Binary Systems
BYEONG-JOO LEE,¹ CHANG-SEOK OH² and JAE-HYEOK SHIM²
1--Materials Evaluation Center, Korea Research Institute of Standards and Science, Yusong P.O. Box 102, Taejon 305-600, Republic of Korea. 2--Department of Metallurgical Engineering and Center for Advanced Materials Research, Seoul National University, Seoul 151-742, Republic of Korea.

KEY WORDS Phase diagram, Sn-In, Sn-Bi, solder, thermodynamic assessment

A thermodynamic evaluation of the Sn-In and Sn-Bi system has been made by using thermodynamic models for the Gibbs energy of the individual phases. An optimized set of thermodynamic parameters was obtained taking into consideration relevant experimental information. The thermodynamic parameters of the Sn-In and Sn-Bi systems and comparisons between calculation and experimental data are presented.

A Numerical Method for Predicting Intermetallic Layer Thickness Developed During the Formation of Solder Joints
MATHEW SCHAEFER,¹ WERNER LAUB,¹ JANET M. SABEE,¹ RAYMOND A. FOURNELLE,¹ and PING S. LEE²
1--Department of Mechanical and Industrial Engineering, Marquette University, Milwaukee, WI 53233. 2--Allen-Bradley Company, Milwaukee, WI 53204.

KEY WORDS Cu-Sn intermetallics, Cu substrate, intermetallic compound growth, Sn-Pb-Ag solder

A numerical method has been developed for calculating the thickness of intermetallic layers formed in substrate-solder systems during the soldering process. As input, the method requires the temperature-time profile for the soldering process and the isothermal liquid state growth rate parameters for the growth of the intermetallic layer. These usually consist of a growth constant, k₀, and an activation energy, Q. The method allows one to predict the thickness of a layer at any time during the soldering process. As such, it can be used in industrial solder processing to enhance the reliability and lifetime of solder joints by allowing control of the thickness of intermetallic layers. The validity of the method is demonstrated for intermetallic growth between copper and 62Sn-36Pb-2Ag solder. The kinetic parameters for the chosen model system were experimentally determined and isothermal intermetallic layer growth between molten solder and copper was found to follow a t^0.25 dependence on time t. The growth rate increased with increasing temperature according to an Arrhenius dependence in the temperature range 187 to 258°C with Q = 7.04 kJ/mol and k₀ = 7.75 µm/min^0.25.

A Study of Parasitic Reactions Between NH₃ and TMGa or TMAl
C.H. CHEN,¹ H. LIU,¹ D. STEIGERWALD,¹ W. IMLER,¹ C.P. KUO,¹ M.G. CRAFORD,¹ M. LUDOWISE,² S. LESTER,² and J. AMANO²
1--Hewlett-Packard Optoelectronics Division, San Jose, CA 95131. 2--Hewlett-Packard Labs, Palo Alto, CA 94304.

KEY WORDS Aluminum nitride, gallium nitride, parasitic reaction, pressure dependence

The growth of AlGaN using organometallic vapor phase epitaxy has been studied as a function of reactor pressure in a horizontal reactor. At atmospheric pressure, GaN with growth efficiency comparable to that of GaAs in the same reactor is obtained. In addition, the GaN growth efficiency changes little at different reactor pressures. These results indicate that the parasitic reaction between TMGa and NH₃ is not substantial in the reactor used in this study. On the other hand, AlN growth at atmospheric pressure has not been possible. By lowering the reactor pressure below 250 Torr, AlN deposition is achieved. However, the growth efficiency decreases at higher reactor pressures and higher growth temperatures, indicating that a strong parasitic reaction occurs between TMAl and NH₃. For the ternary AlGaN, lower pressure also leads to more Al incorporation. The results indicate that parasitic reactions are much more severe for TMAl+NH₃ than for TMGa+NH₃.

Influence of Ga vs As Prelayers on GaAs/Ge Growth Morphology
Q. XU,¹ J.W.P. HSU,² E.A. FITZGERALD³ J.M. KUO,⁴ Y.H. XIE,⁴ and P.J. SILVERMAN⁴
1--Department of Physics, University of Virginia, Charlottesville, VA 22901. 2--Department of Physics, University of Virginia, Charlottesville, VA 22901. e-mail: jhsu@Virginia.edu 3--Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. 4--AT&T Bell Labs, Murray Hill, NJ 07974.

KEY WORDS GaAs/Ge, molecular beam epitaxy (MBE), prelayers, surface morphology

The surface morphology of GaAs films grown on Ge substrates is studied by scanning force microscopy. We find a dramatic difference arising from Ga as opposed to As prelayers in the formation of anti-phase boundaries (APBs), surface features near threading dislocations, and surface roughness, for films as thick as 1 µm. Ga prelayer samples are smooth; thin films display some APBs with predominantly one growth domain while the 1 µm thick film displays the morphology of a homoepitaxial GaAs film. In contrast, As prelayer samples are rough with complicated APB structures, which can be attributed to the increase in single steps during As₂ deposition.

N-Type Hg_1-xCd_xTe: Undoped x=0.3 LPE Material for SPRITE IR Detectors
A. MC ALLlSTER,^1,2 E.S. O'KEEFE,¹ P. CAPPER,¹ F.A. CAPOCCI,¹ S. BARTON,^1,3 and D.T. DUTTON^1,4
1--GEC-Marconi Infra-Red Limited, Millbrook Industrial Estate, Southampton SO15 0EG, UK. 2--Present address: Post Production & Graphic Design, Dept, BBC TV Centre, Wood Lane, London W12 7RJ, UK. 3--Present address: 17 Blackwater Mews, Totton, Southhampton SO40 2GL, UK. 4--Present address: DRA Malvern, St. Andrews Road, Great Malvern, Worcestershire WR14 3PS, UK.

KEY WORDS HgCdTe, infrared detectors, Hg1-xCdxTe (MCT), liquid phase epitaxy (LPE), n-type, SPRITES

The requirement for two color SPRITE detectors, with elements sensitive in the ranges 3-5 µm (MW) and 8-14 µm (LW) at 77K, is met using Hg_1-xCd_xTe elements of composition x=0.3 and x=0.2, respectively. The need for low defect levels for increased performance indicates the use of liquid phase epitaxy (LPE). While LW material is fairly well characterized, the growth and conversion to n-type of MW LPE has proved more difficult. Reported work shows limited data and limited success in converting MW LPE to n-type, and this primarily in donor-doped material. This paper describes the growth, annealing to n-type and characterization of Hg_0.7Cd_0.3Te. High n-type conversion yields were obtained, with low donor levels (mid-10¹³ to mid-10¹⁴ cm^-3), high mobility (>10⁴ cm² (Vs)^-1) and long minority carrier lifetime (>10 µs).

LETTERS SECTION

KEY WORDS Field screening, gas-source molecular beam epitaxy (GSMBE), InAsP/InP, InP (111)B substrate, piezoelectric effect

Low-temperature photoluminescence measurements were performed on InAsP/InP strained quantum wells grown on InP (111)B substrates by gas-source molecular beam epitaxy. The emission energy was observed to increase as the pump-power density increased. This was attributed to the screening of the internal piezoelectric field by photo-generated carriers. The energy shift was as large as 35 meV for an InAs_0.28P_0.72/InP quantum well with a lattice mismatch of ~0.9%. A similar structure with a smaller strain showed saturation of the energy shift with increasing pump-power density. We performed a model calculation which includes the quantum confined Stark effect, and this saturation was correlated with a flat-band structure of the quantum well due to the nearly complete screening of the built-in electric field.

EDITORIAL ANNOUNCEMENT

Original manuscripts with original figures suitable for reproduction, two copies of the manuscript, and a completed copyright form should be submitted to one of the Special Issue editors.

Professor Ilesanmi Adesida
Microelectronics Laboratory University of Illinois
208 N. Wright Street
Urbana, IL 61801
Phone: 217/244-6379
Fax: 217/244-6375
email: adesida@capone.ccsm.uiuc.edu

Professor Isamu Akasaki
Department of Electrical and Electronic Engineering
Meijo University
1-501 Shiogamaguchi
Tempaku-ku, Nagoya 468 Japan
Phone: 81-52-832-1151 (ext. 5089)
Fax: 81-52-832-1244
email: Akasaki@meijo-u.ac.jp

Professor Hiroyuki Matsunami
Department of Electrical Engineering
Kyoto University
Yoshidahonmachi, Sakyo
Kyoto 606-01 Japan
Phone: 81-75-753-5340
Fax: 81-75-751-1576
email: matsunam@kuee.kyoto-u.ac.jp

Professor Michael R. Melloch
1285 Electrical Engineering Building
Purdue University
West Lafayette, IN 47907-1285
Phone: 317/494-3528
Fax: 317/494-6441
email: melloch@ccn.purdue.edu